Deformation induced evolution of plasmonic responses in polymer grafted nanoparticle thin films.

Multi-functional nanoparticle thin films are being used in various applications ranging from biosensing to photo-voltaics. In this study, we integrate two different numerical approaches to understand the interplay between the mechanical deformation and optical response of polymer grafted plasmonic nanoparticle (PGPN) arrays. Using numerical simulations we examine the deformation of thin films formed by end-functionalised polymer grafted nanoparticles subject to uniaxial elongation. The induced deformation causes the particles in the thin film network to rearrange their positions by two different mechanisms viz. sliding and packing. In sliding, the particles move in the direction of induced deformation. On the other hand, in packing, the particles move in a direction normal to that of the induced deformation. By employing a Green's tensor formulation in polarizable backgrounds for evaluating the optical response of the nanoparticle network, we calculate the evolution of the plasmonic response of the structure as a function of strain. The results indicate that the evolution of plasmonic response closely follows the deformation. In particular, we show that the onset of relative electric field enhancement of the optical response occurs when there is significant rearrangement of the constituent PGPNs in the array. Furthermore, we show that depending on the local packing/sliding and the polarization of the incident light there can be both enhancement and suppression of the SERS response.

Mechanical response of networks formed by end-functionalised spherical polymer grafted nanoparticles.

Via computer simulations we examine the mechanical response of hybrid polymer-particle networks composed of rigid spherical nanoparticles with long flexible polymer chains grafted onto their surface. The canopy of grafted polymer arms are end-functionalised such that interacting polymer-grafted nanoparticles (PGNs) form labile bonds when their coronas overlap. In the present study, the number of grafted arms, f, are such that the PGN brushes are in the small (f = 600) and intermediate curvature (f = 900 and 1200) regime with stable bonded interactions. To investigate the mechanical response of networks formed by these PGNs, controlled uniaxial elongation at a specified pulling rate is imposed on a 2-D network of PGNs placed on a hexagonal lattice. In the simulations, the force required to deform the network is measured as a function of the elongation and pulling rate imposed on the network until the network fails. By analysis of the force-strain curves and the rearrangement of the PGNs in the network we show that an increase in the number of grafted arms, pulling velocity and energy of the bonded interactions alters both the toughness and the mode of failure of the networks. In particular, we show that an increase in the number of grafted arms results in a reduction of toughness. Furthermore, analysis of the simulations of force relaxation after rapid extension indicates that the relaxation in deformed networks can be characterised by one or two time scales that depend on the number of grafted arms. The analysis of force-strain curves and force relaxation demonstrate the role of Deborah number, De, and the limitations in the use of a unique De in understanding the mechanical response of the networks respectively.